1. Field of the Invention
The present invention relates to a hydraulic control apparatus for a hydraulically operated power transmitting system for automotive vehicles having a fluid coupling which incorporates a lock-up clutch.
2. Discussion of the Prior Art
In the art of an automotive vehicle, there is known a hydraulically operated power transmitting system in which power is transmitted from an engine to a transmission via a fluid coupling which is equipped with a lock-up clutch. In this type of vehicle power transmitting system, a hydraulic control apparatus includes a lock-up clutch control valve which is adapted to switch the lock-up clutch between its engaged position and its released or disengaged position, by feeding a working fluid into one of the engaging and releasing chambers formed within the fluid coupling. An example of this type of hydraulic control apparatus is disclosed in laid-open Publication No. 61-45167 of unexamined Japanese Patent Application.
The lock-up clutch is generally provided to eliminate a power loss caused by the power transmission by the fluid coupling. In this respect, it is desirable that the lock-up clutch be engaged a comparatively short time after the vehicle is started or after the transmission is shifted from one position to another. To this end, the lock-up clutch is controlled such that the clutch is engaged when the vehicle running speed exceeds a predetermined reference value, and released when the vehicle speed is lowered below the reference value. This reference value is determined to be as low as possible to the extent that assures normal running of the vehicle. This arrangement, however, may cause an undesirable situation in which the drive wheels of the vehicle are brought to a stop before the lock-up clutch is fully released, whereby the engine may stall upon stopping of the vehicle. This situation occurs when an abrupt brake is applied to the vehicle or the vehicle is braked during running on a road surface having a low friction coefficient.